1. Field of the Invention
The invention relates to a method for determining a sprinkler water delivery requirement to control a fire. The invention also relates to an apparatus for measuring heat flux, especially in connection with fire protection, the fire testing of materials, and determining a sprinkler water delivery rate.
2. Background of the Invention
The fire hazard represented by storage of a given material is often characterized by the rate of delivered sprinkler water required to suppress or control a fire of that material. The present state-of-the-art for hazard characterization is to perform replicate tests in which: the material is ignited; the fire is allowed to grow until it is sensed by sprinklers; and the sprinklers then activate to deliver water to the fire. The delivered water density, that is, the amount of water delivered by the sprinklers per unit area of the floor, is systematically increased until a delivery rate that controls the fire is found. Many standard tests must be performed with the same material to determine what rate of sprinkler water delivery is required to control the fire from the burning material. These replicate tests consume a great deal of personnel labor and material, and are thus very expensive and time-consuming.
The severity of fires and the hazards they present are assessed in terms of the total chemical heat release rate of the fire and the heat flux emitted. Heat flux is defined as the rate of energy transfer per unit surface area. Heat flux is typically expressed in units of kilowatts per square meter or BTU per square foot per minute. The measurement of heat flux is of importance in many sciences, including the fire testing of many materials. The heat flux emitted by burning materials may ignite, or aid in the burning of, nearby materials. In one known test set up, a gas burner is positioned at the base of and between two parallel panels on which a test material, for example, a fire resistant material such as polyurethane insulation, is placed. Measurement of heat flux in this parallel panel test provides valuable information about the response of the test material to the flames from the burner.
Instrumentation presently available for measuring heat flux requires complex, time-consuming installation, and is not sufficiently robust to withstand repeated use in very severe fire environments. The conventional instrumentation usually consists of water-cooled heat flux gauges that need to be individually installed, for example, directly on the panels bearing the test material. These gauges are exposed to flames during use. Individual heat flux gauges must undergo time-consuming calibration before and after the test because their sensing elements are easily damaged or impacted by fire impingement and by the deposition of soot and other incomplete products of combustion. A measurement uncertainty arises when post-test calibration shows that the gauge calibration constant has shifted as a result of this impact. Moreover, the gauges are individually water-cooled and mounted to view the flames through openings drilled in the material and supporting structure. This adds time and expense to the testing program and severely limits the number of heat flux measurement stations that can be installed. In some fire test configurations, such as commodity classification, it is not practical to install heat flux gauges due to the difficulty of protecting water cooling lines and electrical connections in highly hazardous locations.
An object of the invention is to provide an inexpensive, easily installed device for measuring heat flux distribution.
Another object of the invention is to provide a simple method for measuring heat flux distribution.
Yet another object of the invention is to provide a durable apparatus for measuring heat flux distribution from gas burners or fire testing apparatuses.
Still another object of the invention is to provide a method for evaluating fire hazards based on measurements of the heat flux in test fires.
Another object of the invention is to provide a method for evaluating the total heat transfer to the burning fuel from spatially distributed heat flux measurements. The total heat transfer is defined as the product of heat flux and the area receiving that heat flux, summed over the entire area receiving heat flux. Total heat transfer is typically expressed in units of kilowatts or BTU/minute.
A further object of the invention is to provide a method for determining the area over which heat is transferred to the material.
Another object of the invention is to provide a method for determining the rate of flow of sprinkler water required to control a fire based on a measurement of the total heat transfer to the burning fuel.
A further object of the inventions is to have a single test that is able to determine the required flow rate of sprinkler water necessary to control a fire for a given material.
The amount of sprinkler water flow rate required to control a fire can be determined by the method of the present invention which includes: measuring the spatial heat flux distribution in a test fire; calculating the effective heat flux received by the material surface, and calculating the sprinkler water delivery rate needed to absorb the heat flux using the energy required to vaporize the delivered water.
The method of the present invention solves the problems of conventional methods by reducing or eliminating the need for multiple and incremental testing. It has been discovered that the sprinkler water delivery rate required for control of the commodity is proportional to the total heat transfer to the fuel (i.e. product of the flame heat flux and flame area) just before the moment when sprinklers sense the fire, causing the sprinkler valve to open and deliver water to the fire. The proportionality constant is easily calculated from the heat of vaporization of water, that is, the rate at which water will be converted to steam per unit of applied heat flux.
By the present invention, the amount of sprinkler water necessary to control the burning of a material can be determined from a single test. The method of the present invention enables an evaluation of the fire hazard of materials based on heat flux measurements. The rate of sprinkler water required to control an array of a burning commodity, such as a commodity in a warehouse, is proportional to the heat flux to the surface of the commodity. The heat flux transferred to the heat flux measurement pipe of the present invention in a free-burning fire is closely related to the water flow rate required to suppress the fire.
Instead of individually installed heat flux gauges fixed to test panels to measure heat flux at various heights in a fire test, the heat flux measurement pipe, or device, of the present invention is a unitary device that has the capability for simultaneous measurements of heat flux along its length. The heat flux measurement pipe of the present invention is extremely stable and rugged, has no moving parts, and is easy to position in a test set up. The heat flux measurement pipe does not need to be connected to the panels bearing the test material. Instead, it can merely be positioned near or between the panels while, for example, being supported on wheeled support.
The heat flux measurement pipe is a water-cooled pipe that makes use of the change in water temperature over a distance along a water passageway, for example, a spiral water passageway, within the pipe. An outer pipe fits tightly over an inner core into which a spiral water passageway is machined. Thermocouples for measuring the temperature of the water in the water passageway are fixed on the core at spaced locations in the water passageway, adjacent thermocouples defining sections of the water passageway between them. At steady-state, the net heat transfer rate to each section of the water passageway can be determined from the product of mass flow rate of water entering or leaving that section and the difference in water temperature between the entrance and exit of that section. Accordingly, only the overall water mass-flow-rate and the water temperature at various locations along the passageway need be measured to determine the spatial distribution of heat transferred to the pipe. The thermocouples measure the temperature of the water at the entrance and exit of each section of the water passageway. By forcing the water to travel in a tight spiral in an annulus between the outer pipe and the core, there is assurance that any spatial non-uniformity will be averaged out, improving heat transfer and resulting in a water temperature that represents the average heat transfer to each pipe section. Heat flux levels can be readily determined from the measured water temperatures and the pipe geometry using known calculations.
The heat flux measurement pipe of the present invention can provide measurements of the heat flux distribution from flames in standard tests to evaluate the fire hazard of typical warehouse commodities and determine the rate of sprinkler water delivery needed to control the fire.
Thus, the use of the heat flux measurement pipe in fires involving test materials measures the spatial distribution of heat flux, and from this the sprinkler water delivery rate that would absorb the heat flux in converting the water to steam can be determined. When the heat flux from the fire is absorbed, the fire will stop spreading and burn out. This method of evaluating fire hazards determines the required sprinkler water delivery rate from heat flux measurements of a single test, rather than from the multiple tests that were heretofore considered necessary.